A scanning sensor system gathers light energy in the ultraviolet, visible, and/or infrared ranges and directs the energy to a sensor. The sensor converts the light energy to electrical signals for subsequent analysis. In one design approach, the sensor remains relatively fixed in its spatial orientation. The light energy is directed to the sensor by a movable scanning mechanism, such as a rotating telescope, which is aimed in a direction of interest. Light energy received from that direction is redirected by the telescope""s optical train, which has at least some movable elements, to the sensor. The optical train may also be designed to magnify or otherwise modify the image.
In one mission, a scanning sensor system is placed on board a platform (such as a spacecraft) that travels above the earth or other heavenly body. The scanning sensor system travels along a track above the surface of the body. The telescope scans laterally (i.e., cross track) so that the sensor system is able to sense a scene along a swath of the surface centered on the track of the platform.
Several designs have been developed for the scanning portion of such a scanning sensor system. A double-sided paddle wheel scan system has a double-sided flat mirror which is rotated about an axis perpendicular to the incident light. This paddle wheel system has the disadvantage that the beam footprint on the scanning mirror varies with scan angle, with the result that it may be difficult to package, it may have a high inertia, and it may require a high torque applied by the scan motor. It may also be difficult to baffle the paddle wheel system against stray light, and there are variations in polarization and overall system response with scan angle. Another design utilizes a single-sided, compound-angle head mirror which is rotated about an axis that may or may not be perpendicular to the incident beam. This configuration produces image rotation with respect to scan angle, which can lead to image registration problems. It may also have a substantial inactive scan period. Another scan approach is a single-sided rotating telescope that has the disadvantages of a long inactive scan period and difficulty in mounting and support.
There is a need for an improved scanning sensor system, and particularly one which has a relatively short inactive period and is readily implemented mechanically. The present invention fulfills this need, and further provides related advantages.
The present invention provides a scanning sensor system, which preferably includes a rotating telescope. A preferred embodiment of the system is well balanced and symmetrical as to the largest moving components. The primary telescope mirrors are placed relatively near to an axis of rotation, reducing the inertia of the scanning telescope assembly. Other benefits relative to available scanning telescopes include reduced polarization variations and response as a function of scan, absence of image rotation, inherent band-to-band registration, improved stray light rejection, limited scene footprint growth as a function of scan angle, and the potential for improved packaging.
In accordance with the invention, a scanning sensor system comprises a light sensor and a scanning telescope. The scanning telescope includes at least two primary telescope mirrors supported (preferably symmetrically) about a primary-mirror rotation axis. Each of the primary telescope mirrors is oriented to receive incident light along an incident ray path that is not parallel to the primary-mirror rotation axis. The scanning telescope includes at least one additional telescope mirror positioned to alternately receive a first reflected light beam from a first one of the primary telescope mirrors and thereafter a second reflected light beam from a second one of the primary telescope mirrors, and to direct the reflected light beam into the light sensor along an optical path. A primary mirror drive rotates the at least two primary mirrors about the primary mirror rotation axis at a primary mirror angular instantaneous rate of rotation. A half-angle derotation mirror is positioned to reflect a light beam in the optical path, and a derotation mirror drive rotates the derotation mirror about a derotation-mirror axis parallel to the primary mirror rotation axis at a derotation angular rate of rotation that is one-half of the primary mirror angular instantaneous rate of rotation during the active scan period. The primary telescope mirrors are preferably mounted in a housing.
In a preferred approach, there are exactly two primary telescope mirrors supported 180 degrees apart around the primary-mirror rotation axis, and the two primary mirrors face in diametrically opposed directions. The incident ray path is perpendicular to the primary-mirror rotation axis. There may be at least two additional telescope mirrors, one of which rotates with the primary telescope mirror and one of which is stationary. The scanning telescope may comprise an anastigmat mirror array. The derotation-mirror axis is preferably coincident with the primary-mirror rotation axis.
The presently most-preferred design has a pair of identical off-axis scanning telescopes, oriented back-to-back in a double-sided, rotating scan mechanism. As the double-sided scanning telescope assembly is rotated about the primary-mirror rotation axis, each scanning telescope alternately collects energy from various portions of the scene and directs it onto a single, rotating half-angle mirror. The half-angle mirror is rotated and moved independently of the rotation of the primary telescope mirrors. During active scan periods, the half-angle mirror is rotated at one-half the instantaneous angular rate of the primary mirrors, which allows the scene energy to be directed to a single-fixed location regardless of scan angle. This configuration requires that the reflecting surface of the half-angle mirror lie parallel to the primary-mirror rotation axis of the primary telescope mirrors. After reflection at the half-angle mirror, the beam is directed into the remaining portion of the optical imaging system and along the optical path to the sensor.